There has been a trend in recent years for a laser light source which is used as a light source in an optical pickup apparatus for reproducing information which has been recorded in an optical disc and for recording information on an optical disc, toward a shorter wavelength. For example, laser light sources with 390-420 nm wavelength, such as a blue-violet semiconductor laser, are reaching the stage of practical application. By using these blue-violet laser light sources, information of 15-20 GB can be recorded on an optical disc with a diameter of 12 cm by using an objective lens with the same numerical aperture (NA) as that for DVD (Digital Versatile Disc), and information of 23-25 GB can be recorded onto an optical disc with a diameter of 12 cm by using an objective lens with increased NA up to 0.85.
As an example of an optical disc using the above-described objective lens with NA 0.85, there is cited a BD (Blu-ray Disc). Since increased comma is generated because of a tilt (skew) of the disc, a BD has been designed so that a protective layer has thinner thickness (which is 0.1 mm, while that of DVD is 0.6 mm) than that of DVD, to reduce the amount of comma caused by the skew.
On the other hand, it is considered that an optical disc player/recorder (optical information recording reproducing apparatus) is worthless as a product when the optical disc player/recorder is capable of recording/reproducing information just for BDs properly. Taking account of a fact that, at present, DVDs and CDs (Compact Discs) storing various kinds of information have been on the market, it is not sufficient that the optical disc player/recorder can record/reproduce information just for BDs, and an attempt providing an optical disc player/recorder capable to record/reproduce information also for DVDs and CDs which have already been owned by users, leads to enhancement of a commercial value of the optical disc player/recorder for BDs. From such the background, an optical pickup apparatus installed in the high-density optical disc player/recorder is required to be capable of appropriately recording/reproducing information not only for BDs but also for a DVDs and a CDs.
As a method by which information can be adequately recorded/reproduced while the compatibility is maintained to anyone of BDs, DVDs and CDs, there can be considered a method to selectively switch an optical system for BDs and an optical system for DVDs and CDs, corresponding to a recording density of an optical disc on which information is recorded/reproduced. However, it is disadvantageous for the size-reduction and increases a cost, because plural of optical systems are needed.
Accordingly, in order to simplify a structure of the optical pickup apparatus and to intend a reduction of its cost, it is preferable to form the optical system for BDs and the optical system for DVDs and CDs into a common optical system, to reduce the number of optical parts forming the optical pickup apparatus as much as possible, even in the optical pickup apparatus with compatibility. Then, providing the common objective lens which is arranged with facing an optical disc, is most advantageous for the simplification of the structure and for cost reduction of the optical pickup apparatus. In order to obtain a common objective lens for plural kinds of optical discs which use different wavelengths for recording/reproducing information, it is required that a diffractive structure having a wavelength dependency for the spherical aberration, is formed in the objective optical system.
Further, it is required that an objective lens commonly used for three types of optical disc of BDs, DVDs, and CDs, can cope with a difference in necessary numerical aperture among BDs, DVDs, and CDs. For example, the objective lens is requested to converge a light flux passing through the almost whole area of an effective optical surface of the objective lens, onto an information recording surface, for BDs with large necessary numerical aperture; and is requested to converge a light flux passing through the central portion of the objective lens, onto an information recording surface and to form a light flux passing through the surrounding portion of the central portion into a flare light so as not to converge the light flux onto an information recording surface, for CDs with small necessary numerical aperture. Therefore, an objective lens commonly used for three types of optical disc of BDs, DVDs, and CDs has an optical surface which is divided into three concentric areas (central area CN for converging light fluxes for BDs, DVDs, and CDs, intermediate area MD for converging light fluxes for BDs and DVD, and peripheral area OT for converging a light flux for BDs), and these areas are formed to exhibit different optical performances.
However, it has been found that such the objective lens hardly maintain its optical performance without occurrence of phase shift for certain two wavelength on the border of the central area and the intermediate area.
With respect to the difficulty, JP-A No. 2008-293630 has provided an objective lens including two basic structures which are overlapped together in the central area and exhibit the same diffraction order, and further including two basic structures which are overlapped together in the intermediate area and exhibit the same diffraction order. Thereby, an objective lens without the problem of the phase shift on the boundary of the central area and the intermediate area, and an optical pickup apparatus equipped with the objective lens are provided.
The inventor has tried to design an objective lens compatibly used for of BDs, DVDs, and CDs, based on an example of an objective lens compatibly used for of HD-DVDs, DVDs, and CDs disclosed in JP-A No. 2008-293630. However, it has been found that unwanted light generated when the objective lens works for a CD affects a converged spot, which makes difficult to record and/or reproduce information in excellent condition.